This invention relates to a head drive mechanism, and more particularly to a drive mechanism for driving a write head for a printer or the like.
A conventional optical printer in which a phosphor head, an LED head or the like is incorporated is generally constructed in such a manner that a printer head is linearly moved to carry out optical writing on a photosensitive paper. Also, in an ink jet printer and a thermal printer as well, a printer head is linearly moved with respect to a recording paper to carry out recording thereon.
Now, a conventional head drive mechanism for such a printer will be described with reference to FIGS. 7 and 8, wherein FIG. 7 is a perspective view showing such a head drive mechanism and FIG. 8 is a schematic view showing a transmission system for transferring a head by means of a wire rope.
In FIG. 7, reference numeral 51 designates a housing 51 for a printer, of which a part is broken away for the sake of brevity. The housing 51 is provided therein with two shafts 53, which are supported at both ends thereof through supports 52 in the housing 51. The housing 51 is also provided therein with a base 63, which is provided at each of both ends thereof with a bearing 54, through which the base 63 is slidably supported on the guide shafts 53.
The housing 51 is so constructed that a photosensitive paper (not shown) is arranged on an upper surface of the housing 51 while keeping a photosensitive surface thereof facing down.
Reference numeral 64 designates a photosensitive head, which is mounted on base 63. The photosensitive head 64 is constructed in the form of an optical writing module. The photosensitive head 64 is formed thereon with a dot block 64a which acts to carry out optical writing on the photosensitive surface of the photosensitive paper. The dot block 64a has a plurality of luminous dots arranged in an offset manner so as to form recording dots. The luminous dots each are constructed so as to emit light of luminance defined depending on a drive signal fed thereto from a drive signal generation section (not shown).
The photosensitive head 64 is constructed so as to travel at a constant speed along the guide shafts 53 by means of a transmission system described hereinafter. Unfortunately, a travel speed of the photosensitive head 64 is often cyclically varied depending on mechanical accuracy of a power drive system. Such a variation tends to cause color shading in an image exposed by the photosensitive head 64.
In view of such a problem, the assignee proposed techniques which permit color shading due to a variation in a travel speed of the photosensitive head 64 to be effectively restrained by suitably setting relationship between an interval Ds between luminous dot rows arranged in an offset manner and a reduction ratio of a drive system for transferring the photosensitive head 64, as disclosed in Japanese Patent Application No.67523/1996.
Now, the drive system disclosed in the Japanese application described above will be described with reference to FIG. 8. The drive system includes a motor 56 mounted on a motor mounting plate 55 fixed to the housing 51 shown in FIG. 7. The motor 56 includes an output shaft 56a, on which a drive pulley 57 is fixed. Also, the housing 51 is mounted therein with a pulley mounting support 60, which is then mounted thereon with a tension adjusting lever 61.
The drive system also includes a driven pulley 59, as well as a wire rope 58 which is arranged so as to extend between the drive pulley 57 and the driven pulley 59 while being wound thereon. The wire rope 58 has both ends caulked together by means of a fitment 58a, resulting in being in the form of an endless belt. The wire rope 58 is fixed to the base 63 through the fitment 58a. The photosensitive head 64 mounted on the base 63 is fixed to the base 63 so as to be moved in association with the wire rope 58.
Actually, the wire rope 58 is wound on the drive pulley 57 plural times, resulting in preventing slippage between the drive pulley 57 and the wire rope 58.
The tension adjusting lever 61 is fixedly mounted on one end thereof with a shaft 61a, which is supported on the pulley mounting support 60 in a manner to be pivotally movable. The tension adjusting lever 61 is mounted on a central portion thereof with a support shaft 61c by caulking, to thereby rotatably support the pulley 59 thereon. The tension adjusting level 61 is formed at the other end thereof with a hole in which a spring 61b is engagedly held at one end thereof. The spring 61b is heldly engaged at the other end thereof with the tension adjusting screw 62 to rotate the tension adjusting screw 62, to thereby adjust tension of the wire rope 58.
Rotation of the motor 56 permits turning force of the motor 56 to be transmitted through a reduction gear (not shown) to the drive pulley 57, leading to rotation of the drive pulley 57. Such rotation of the drive pulley 57 is transmitted to the wire rope 58 to drive it, to thereby slide the base 63 along the guide shafts 53 in directions indicated at arrows in FIG. 7, resulting in the photosensitive head 64 on the base 63 being likewise slid in the directions of the arrows or a scan direction thereof.
The drive system employs a wire drive system including the drive pulley 57 and wire rope 58 arranged at a final stage of the power transmission for transferring the head in the scan direction, to thereby eliminate nonuniform rotation of the reduction gar and drive pulley 57, resulting in ensuring traveling of the head at a uniform speed.
Unfortunately, the wire drive system constructed so as to ensure smooth power transmission has such a disadvantage as described hereinafter.
More specifically, it requires operation of subjecting both ends of the wire rope to caulking to render the rope endless. Also, it needs to wind the wire rope on the drive pulley plural times during assembling of the drive system, to thereby prevent slippage of the wire rope on the drive pulley. Thus, the drive system causes a deterioration in workability. Further, a reduction in diameter of the drive pulley for small-sizing of the printer requires to use a thin wire in view of restriction on an allowable bending radius of the wire. Unfortunately, this causes the wire to tend to have a bending propensity, leading to a further deterioration in workability.
In addition, in order to ensure that the pulley exhibits increased durability, the wire rope must be coated with nylon. To this end, careful attention must be paid to keep the coating from being damaged during assembling of the drive system, so that workability in the assembling may be further deteriorated.
Also, when the printer is constructed into a portable type, the head drive mechanism is required to be constructed so that the photosensitive head is moved in a vertical direction as well. Unfortunately, this causes load to be increased as compared with horizontal movement, so that slippage tends to occur between the drive pulley and the wire rope. In order to avoid such a disadvantage, it is needed to arrange a motor increased in output and increase both tension of the wire rope and the number of turns of the wire rope on the drive pulley, to thereby increase slip torque. However, this renders assembling of the printer highly troublesome and increases the number of steps in the assembling, leading to an increase in manufacturing cost.
In view of the foregoing, it would be considered to employ such a toothed belt as incorporated in a miniature precision machine. Such a toothed belt generally goes by the name of a timing belt in the art. Unfortunately, incorporation of such a toothed belt in the printer causes an important problem. More particularly, traveling of the toothed belt often causes a variation or nonuniformity in travel speed of the toothed belt wherein time required for the toothed belt to travel by a distance corresponding to an interval or pitch between each adjacent two teeth of the toothed belt constitutes a cycle. Such a phenomenon would be due to flexibility of the toothed belt.
Such a toothed belt is constructed in such a manner as shown in FIG. 9. The toothed belt designated at reference numeral 70 is wound on a toothed pulley 71. The toothed belt 70 is constructed by integrally forming a flat belt material with teeth 70a at predetermined pitches P. The flat belt member has a core which is made of a glass fiber incorporated therein to exhibit satisfactory tensile strength and regulate elongation of the belt in a prescribed standard. Also, the flat belt member is constructed so as to exhibit flexibility sufficient to permit it to be satisfactorily wound on the toothed pulley 71.
Transmission of rotation of the toothed pulley 71 to the toothed belt 70 is mainly carried out by contact between side surfaces of each of the teeth 70a of the toothed belt 70 and those of each of teeth 71a of the toothed pulley 71. The transmission is also carried out by friction due to abutment of an inner surface 70b of a flat belt portion (or a portion of the flat belt member defined between each adjacent two teeth 70a of the toothed belt 70) against an arcuate portion of an addendum of each of the teeth of the toothed pulley 71.
The teeth of the toothed belt 70 and toothed pulley 71 are formed into a reduced thickness, to thereby cause a gap between the tooth of the toothed belt 70 and that of the toothed pulley 71 to be increased when the toothed belt 70 is wound on the toothed pulley 71.
Further, in order to prevent vibration of the toothed belt 70 during operation of the printer and ensure conformability between the toothed belt 70 and the toothed pulley 71, it is desired that initial tension is applied to the toothed belt 70. However, in order to reduce loss during power transmission, actually the toothed belt 70 is typically operated while keeping initial tension at a sufficient level from being applied thereto when it is microminiaturized. This fails to permit the flat belt portion and tooth of the toothed belt to be fully bent into an arcuate shape to conform to the toothed pulley, as indicated at reference character A in FIG. 9. Rather, the flat belt portion decreased in thickness tends to be bent into a dogleg-like shape as indicated at B and the tooth tends to be kept flat as indicated at C. As a result, the flat belt portion is increased in length or width as compared with a length of the arcuate portion of the tooth of the toothed pulley 71 and the tooth of the toothed belt 70 is reduced in length or width as compared with that of a recessed portion of the toothed pulley defined between each adjacent two teeth of the toothed pulley.
This causes the toothed belt 70 to fail to travel at a uniform speed in spite of rotation of the toothed pulley 71 at a uniform speed. Such a variation or nonuniformity in travel speed of the toothed belt repeatedly occurs for every tooth, leading to color shading on a printed image.